1. Field of the Invention
This invention relates to systems and methods for extending a detection range of an inspection system used for inspecting a specimen. More specifically, the present invention relates to systems and methods for extending the detection range of an inspection system by forcing the photodetector into the non-linear range.
2. Description of the Related Art
The following descriptions and examples are given as background only.
Fabricating semiconductor devices, such as logic, memory and other integrated circuit devices, typically includes processing a specimen such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that typically involves transferring a pattern to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process, and thus, higher profits. Inspection has always been an important part of semiconductor fabrication. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices. For instance, detecting defects of decreasing size has become increasingly necessary, since even relatively small defects may cause unwanted aberrations in the semiconductor device, and in some cases, may cause the device to fail.
Many different types of inspection tools have been developed for the inspection of semiconductor wafers, including optical and E-beam systems. Optical inspection tools may be generally characterized into dark-field and bright-field inspection systems. Dark-field systems typically provide a higher detection range than bright-field systems. For instance, dark-field systems detect the amount of light scattered from the surface of a specimen when an incident beam is supplied to the specimen at a normal or oblique angle. The amount of scattered light detected by the system generally depends on the optical characteristics of the spot under inspection (e.g., the refractive index of the spot), as well as any spatial variations within the spot (e.g., uneven surface topologies). In the case of dark-field inspection, smooth surfaces lead to almost no collection signal, while surfaces with protruding features (such as patterned features or defects) tend to scatter much more strongly (sometimes up to six orders of magnitude or more). On the other hand, bright-field inspection systems direct light to a specimen at a particular angle and measure the amount of light reflected from the surface of the specimen at a similar angle. In contrast to dark-field systems, the detection range of a bright-field system is generally no more than about two orders of magnitude.
Most inspection tools are designed for inspecting either unpatterned or patterned semiconductor wafers, but not both. Since the tools are optimized for inspecting a particular type of wafer, they are generally incapable of inspecting different types of wafers for a number of reasons. For example, most unpatterned wafer inspection tools are configured such that all of the light collected by a lens (or another collector) is directed to a single detector, which generates a single output signal representative of all of the light collected by the lens. Although sufficient for unpatterned wafer inspection, a single detector inspection tool is generally incapable of inspecting patterned wafers.
When used for patterned wafer inspection, the light supplied to a single detector may include light scattered from patterns or features on the patterned wafer in addition to other scattered light (e.g., light scattered from defects). In some cases, the single detector may become saturated, and as a result, may not yield signals that can be analyzed for defect detection. Even if the single detector does not become saturated, the light scattered from patterns or other features on the wafer cannot be separated from the other scattered light thereby hindering, if not preventing, defect detection based on the other scattered light.
For this reason, many patterned wafer inspection tools employ at least two detectors for improved spatial resolution and detection range. An approach of this sort is described by Almogy et al. in U.S. Patent Application Publication No. 2003/0058433, whose disclosure is herein incorporated by reference. Almogy describes an inspection system that utilizes at least two detectors with separate detection channels. One of the detectors is optimized for high resolution, while the other is designed with a high saturation level to improve detection range, typically at the expense of resolution. The light scattered from a specimen is split among the detectors with the addition of various optical components. Although Almogy may improve spatial resolution and detection range, Almogy does so by requiring multiple detectors with additional optics and electronic circuitry, all of which consume additional space, increase complexity, and incur higher cost.
In some cases, one or more of the detectors may become saturated, especially when imaging with a dark-field system. As noted above, dark-field scattering signals obtained from a patterned wafer may vary by six orders of magnitude (or more) due to the variation in surface topology from smooth surface regions (which appear dark) to highly textured regions (which appear bright). It is often difficult, especially with detection systems operating at high data rates, to collect meaningful signals from both the very dark and the very bright areas of the wafer without “on-the-fly” gain adjustment.
“On-the-fly” gain adjustment is another method commonly used to improve spatial resolution and/or detection range. One such method is described by Wolf in U.S. Pat. No. 6,002,122, whose disclosure is herein incorporated by reference. In the method described by Wolf, the output signal from a photomultiplier tube (PMT) is processed by a logarithmic amplifier and gain correction mechanism. The logarithmic amplifier and gain correction mechanism provides a feedback signal to the PMT for adjusting the detector gain “on-the-fly” (e.g., by changing the bias potentials supplied to the dynodes) to account for variations in light supplied to the detector. When larger amounts of light are supplied to the detector, the PMT gain may be reduced to avoid anode saturation, and therefore, extend the detection range. On the other hand, the PMT gain may be increased when smaller amounts of light are supplied to the detector to improve spatial resolution in the low signal range. However, “on-the-fly” gain adjustment tends to increase the level of noise and limit the sensitivity in the low signal range, and requires highly trained personnel to operate the complex and expensive drive electronics.
Therefore, a need remains for improved inspection systems and methods for extending the detection range of a wafer inspection system. Preferably, such improved systems and methods would provide significant detection range extension without the complexity and cost of real-time gain adjustment, as required by Wolf, or the additional detectors, optics and electronic circuitry required by Almogy. In addition, an improved inspection system would extend the detection range without sacrificing sensitivity, resolution or noise performance (especially in the low signal range). In some cases, the improved inspection system may be used for inspecting both patterned and unpatterned wafers.